Sound barrier

ABSTRACT

A sound barrier made from recycled concrete aggregate, the sound barrier being divided into at least two or more sections, each said section including: an inner layer for supporting the barrier, and an outer layer, said outer layer having a predetermined number of voids for absorbing sound energy at a particular frequency, wherein said respective outer layers of said at least two or more sections have different predetermined numbers of voids so as to absorb sound energy at different frequencies. A method of making a sound barrier is also provided.

[0001] This invention relates to sound barriers. In particular, theinvention relates to concrete sound barriers for highways and other roadsurfaces carrying large volumes of motorised traffic. Anotherapplication of the invention is the use of the sound barrier forinternal or external cladding in buildings.

[0002] City communities are more aware of significant noise coming fromroad and rail traffic. Traffic noise is recognised as seriousenvironmental problem and consequently relevant traffic and governmentauthorities have defined residential noise limits for newly constructedand in (some instances) existing roads. In Denmark for example, thelegal limit is 55 dB(A) day and night, and in Netherlands the nightlimit is even lowered to 45 dB(A). In the state of Victoria inAustralia, the road traffic noise limit is set to 63 dB(A) L10(18 hour).There is often a difference in the sound pressure level generated on theroad surface and what affected residents are willing to accept.

[0003] To reduce the sound pressure level that reaches nearby residents,a wide range of options are available to city authorities to consider,such as earth mounds, wider road reserves, etc. However, spacelimitations and financial considerations dictate in most cases the useof the noise barriers. The maximum practical reduction that can beachieved with the use of a vertical barrier is usually 15 dB(A). Ingeneral, the lower the noise level to be achieved, a higher and thickersound barrier is required. Because of the limited distance betweenbarriers and/or source and receiver, the use of reflective barriers isconsidered to be satisfactory. Consequently, this approach creates morevisual obstruction as well as causing extensive use of materials andsubstantial construction costs. Moreover, these types of barriers, whichreflect or disperse noise, in many cases do not properly satisfactorilysolve traffic noise problems. To lessen the visual impact of thebarriers and to achieve the target noise reduction, the use of quietroad surfaces and sound absorptive barriers have been observed as aneffective alternative.

[0004] In addition to the increased nuisance related to the trafficnoise, another environmental problem that needs immediate attention ofthe community is the growing amount of waste generated and requiringdisposal at local landfill sites. The construction and demolition (C&D)industry, with its waste, is the major contributor to the total wastestream, and accounts for up to 70% of the total. The major component ofC&D waste is concrete, which amounts for approximately 1.5 milliontonnes per year in the Metropolitan Melbourne area of Victoria,Australia alone. Approximately 50% of concrete waste can be recycled andreused in new road infrastructure projects mainly as a substitute fornatural crushed aggregate in bound and unbound pavement sub-base layers.This recycled concrete waste may be used as an alternative constructionmaterial and is often referred to as recycled concrete aggregate or RCA.RCA materials may be manufactured in quality assured processes to setspecifications, and subsequently have well defined basic engineeringproperties. Concrete products made from RCA are also referred to asrecycled aggregate concrete or RAC. However, RCA and RAC products areoften under utilised.

[0005] According to one aspect of the invention there is provided asound barrier made from recycled concrete aggregate, the sound barrierbeing divided into at least two or more sections, each said sectionincluding:

[0006] an inner layer for supporting the barrier, and

[0007] an outer layer, said outer layer having a predetermined number ofvoids for absorbing sound energy at a particular frequency,

[0008] wherein said respective outer layers of said at least two or moresections have different predetermined numbers of voids so as to absorbsound energy at different frequencies.

[0009] There is also provided according to another aspect of theinvention, a method of making a sound barrier divided into at least twoor more sections from a concrete mix comprising recycled concreteaggregate, the method comprising the steps of:

[0010] (a) casting the concrete mix;

[0011] (b) compacting the concrete mix to form a plurality of voids inthe concrete mix;

[0012] (c) curing the concrete mix to form one said section such that anouter layer of said section has a predetermined number of said voids forabsorbing sound energy at a predetermined frequency;

[0013] (d) repeating said steps (a)-(c) for one or more further sectionswherein the number of voids in each respective outer layer differs so asto vary the frequency of sound energy absorbed by each respective outerlayer, and

[0014] (e) connecting said section and said one or more further sectionsto form said sound barrier to absorb sound energy at differentfrequencies.

[0015] As the outer layer of each sound barrier section has a porousstructure due to the voids, sound energy is absorbed by each section bychannelling sound energy through the voids, causing the sound todissipate or disperse in the structure of each respective outer layer.Accordingly, by varying the number of voids in each sound barriersection, sound at different frequencies is absorbed by the sections ofthe sound barrier. Therefore, this improves the sound absorptioncapacity and allows the sound barrier and method according to theinvention to have an overall smaller thickness and be less visible thanexisting sound barriers. In addition, the sound barrier and methodaccording to the invention reduces the consumption of building materialsby utilising RCA.

[0016] Preferably, the number of voids is determined by varying thethickness of the outer layer. The thickness of the outer layers may beup to 125 mm. Preferably, the thickness of the outer layer is no morethan 85 mm. In a preferred embodiment, the thickness of the outer layeris between 15 mm and 60 mm.

[0017] The outer layers preferably absorb sound energy of frequencies inthe range of between 100 Hz and 2000 Hz.

[0018] It is preferred that there is a transition zone between the innerlayer and the outer layer. The transition zone preferably reflects anyresidual sound energy that travels through the outer layer.

[0019] The inner layer preferably is denser than the outer layer.

[0020] The sound barrier preferably has the three sections arrangedsequentially from top to base relative to the ground in ascending orderof each frequency absorbed or reflected. In a preferred embodiment thesound barrier of the invention has a top section or panel for absorbingsound at 400 Hz, a middle panel for absorbing sound at 600 Hz, and abase panel for absorbing sound at 800 Hz.

[0021] It is preferable that the outer layer has a higher thickness at atop section of the sound barrier than the outer layer at the base of thesound barrier, relative to the ground. The top section outer layerpreferably has large interconnected air channels. The thickness of theouter layer may gradually increase from the base of each sound barriersection to the top portion. Preferably, the top section of the soundbarrier includes up to 15% of the total volume of the sound barrier.

[0022] Preferably, the voids may include one or more of the followingtypes or a combination of the following types: gel-like pores;capillary-like pores; interconnected permeable voids; isolated airpockets or channels, and interconnected air pockets or channels. Thevoids may range in size between 100 Angstroms and 5 mm in diameter.

[0023] The outer layer of each section is preferably covered with anadditional layer, the additional layer having perforations ordiscontinuous portions to facilitate the absorption of sound energy. Theperforations or discontinuous portions preferably extend into theadditional layer to a predetermined depth. The perforations ordiscontinuous portions may extend into the additional layer to a depthsuch that portions of the outer layer are exposed. Preferably, theperforations or discontinuous portions are formed over at least 30% ofthe outer surface of the additional layer. The perforations ordiscontinuous portions can be mechanically induced in the additionallayer.

[0024] The perforations or discontinuous portions can be formed in aregular pattern. One embodiment has 9 mm holes spaced at 20-25 mmcentres. Irregular patterns of perforations or discontinuous portionscan be used.

[0025] The additional layer may be composed of a perforated material. Inone embodiment, the additional layer is made from a mortar including RCAfines, cement, water and an air entraining agent. Other perforatedmaterial may include plasterboard or render.

[0026] The additional layer is preferably thin relative to the thicknessof the inner layer and the outer layer. It is preferred that thethickness of the additional layer is at least 10 mm. It is expected thatthe maximum thickness is at most 25 mm.

[0027] The concrete mix preferably includes one or more of thefollowing: general purpose cement; fly ash; concrete sand and water.Preferably, the concrete mix comprises between 45 and 60% of RCA.

[0028] Preferably, an inner layer is formed with the outer layer. Theinner and outer layers of each sound barrier section can be formed in asingle pour of concrete mix.

[0029] It is preferable that the recycled concrete aggregate has a lowerdensity than normal concrete. The recycled concrete aggregate may have adensity that is 10% lower than the density of normal concrete.

[0030] Preferably, the casting step is performed on a horizontal castingbed.

[0031] It is preferred that the compacting step includes vibrating theconcrete mix at a predetermined frequency and time. The concrete mix ispreferably vibrated at a frequency of 50 Hz for 3 to 6 minutes, morepreferably 3.3 to 5.5 minutes.

[0032] The curing step is preferably performed by steam curing or byintroducing a curing compound to the concrete mix.

[0033] While the perforations or discontinuous portions can be formed inthe additional layer before or after the additional layer is joined tothe outer layer of the second barrier, it is preferred that theperforations/discontinuous portions are formed after the additionallayer is joined to the outer layer.

[0034] It is preferred that the total thickness of the sound barrier is150 mm.

[0035] To assist in the understanding of the invention, preferredembodiments of the invention will now be described with reference to thedrawings, of which:

[0036]FIG. 1 is front view of a sound barrier made according to oneembodiment of the invention;

[0037]FIG. 2 is a sectional side view of the sound barrier of FIG. 1along the line A-A;

[0038]FIG. 3 is a side view of a section of the sound barrier of FIG. 1;

[0039]FIG. 4 is a schematic drawing showing the sound absorbingmechanisms employed by the sound barrier of FIG. 1.

[0040]FIG. 5 is a schematic side view of a section of a sound barrier inaccordance with another embodiment of the invention.

[0041]FIG. 6 is a chart comparing the sound absorbing characteristics ofembodiments of the invention.

[0042]FIG. 1 illustrates a sound barrier 10 in accordance with apreferred embodiment of the invention. Sound barrier 10 comprises threeseparate sections or panels 13, 14, 15 mounted on rigid supports 12. Thesupports 12 are affixed to the ground via footing members 11 made fromRAC. Each of the panels 13, 14, 15 respectively has an outer layer (13a, 14 a or 15 a) and an inner layer (13 b, 14 b or 15 b). The outerlayers 13 a, 14 a, 15 a each have voids for absorbing sound energy. Thethicknesses of the outer layers 13 a, 14 a, 15 a increase from the baseof the sound barrier 10 to the top, as can be seen more clearly in FIG.2.

[0043]FIG. 3 shows a side view of section 13 of sound barrier 10 ofFIGS. 1 and 2 in more detail. Sections 14, 15 have the same structure assection 13 except in relation to the thicknesses of their respectiveinner and outer layers. Section 13 has an inner dense layer 13 b (alsoindicated as “RL”—reflective layer) and outer porous layer 13 a (alsoindicated as “SA”—sound absorbent layer). The inner dense layer 13 bprovides structural support for 13 as well as for reflecting soundenergy at the transition zone 20 between the inner layer 13 b and theouter layer 13 a. The outer layer 13 a has a number of voids in the formof interconnected air channels 40. The outer porous layer 13 a is anabsorptive layer for channelling sound energy through voids or airchannels 40 to disperse or dissipate sound energy generated by trafficnoise.

[0044] It has been found that the greater the porosity of the outerlayer of the sound barrier, the lower the frequency of the soundabsorbed by the sound barrier. This means that the number of voids inthe outer layer determines the frequencies at which sound is absorbed.Accordingly, particular sound frequencies may be absorbed according tothe thickness of the outer porous layer of the sound barrier as thisdetermines the number of voids.

[0045] Thus, each panel or section 13, 14, 15 will absorb and reflectdifferent frequencies of sound. For example, panel 13 has an outer layer13 a of approximately 60 mm thickness and absorbs sound frequencies of400 Hz. Panel 14 has an outer layer 14 a of approximately 40 mmthickness and absorbs frequencies of 600 Hz. Panel 15 has an outer layer15 a of approximately 30 mm thickness and absorbs frequencies of 800 Hz.

[0046] It has been found that sound barriers made in accordance with theinvention may absorb sound having frequencies in the range of between100 Hz and 2000 Hz. In particular, the most effective range offrequencies absorbed is between 400 Hz and 800 Hz. The thickness of theouter layer is preferably no more than 85 mm, although thicknesses of upto 125 mm may be used.

[0047] By dividing the sound barrier into sections or panels, each panelhaving an outer layer of a different thickness, the sound barrieraccording to this preferred embodiment of the invention may absorb abroader range of frequencies than a sound barrier having an outer porouslayer of uniform thickness.

[0048] The sound absorbing mechanisms used by each section 13, 14, 15are illustrated in FIG. 4. Incident sound generated from passing trafficis absorbed or reflected by the sound barrier in two ways—flowresistivity and resonance. Firstly, an incident sound wave may be simplyreflected from the outer porous layer 13 a. Secondly, an incident soundwave may penetrate and be absorbed by the outer layer 13 a. In thiscase, the incident sound wave travels through the outer porous layer 13a via voids or air channels 40, dissipating most, if not all, of itsenergy. Any residual sound energy that reaches transition zone 20 isreflected back through the outer porous layer 13 a due to the densernature of the inner layer 13 b. The reflected residual sound energy willdissipate as it travels back through the outer layer 13 a. Therefore,each sound barrier section 13, 14, 15 both absorbs and reflects soundenergy generated by traffic noises.

[0049] It has been discovered that the sound absorption properties ofthe RAC sound barrier are proportional to the porosity of the RCA thatis used to make up the RAC. This means that the overall porosity of thesound barrier (that is, the concrete density and the number of voids pervolume (also known as void volume)) determines the effectiveness of thesound barrier. Table 1 indicates examples of this relationship betweenporosity of the RCA and noise reduction, the Noise Reduction Coefficientbeing an arithmetic mean of Sound Absorption Coefficient measured at 250Hz, 500 Hz, 1000 Hz and 2000 Hz. TABLE 1 Acoustic characteristics ofsound barrier panels Concrete Noise Density % Reduction Panel [kg/m³]Porosity Coefficient Alpha-400 1,650 22.0 0.42 Alpha-600 1,745 18.0 0.27Alpha-800 1,865 13 0.21

[0050]FIG. 5 schematically shows one section 50 of a sound barrieraccording to another embodiment of the invention. The sound barriersection 50 of this embodiment has an inner structural layer 51, a porousouter layer 52 and an additional or second outer layer 54 covering outerlayer 52. The inner layer 51 and outer layer 52 are respectively thesame as the inner layer 13 a, 14 a, 15 b and outer layer 13 b, 14 b, 15b of the sound barrier 10 of FIG. 1. The only difference between the twoembodiments is the provision of additional layer 54.

[0051] The additional layer 54 has a number of perforations 56 formedover at least 30% of its outer surface 58 to assist in the absorption ofsound energy. The perforations 56 extend from outer surface 58 rightthrough additional layer 54 to expose portions of outer layer 52. It hasbeen found that this additional layer 54 assists in improving the soundabsorbing characteristics of the sound barrier section 50, particularlyat lower frequencies of sound energy.

[0052] In the preferred embodiment, the perforations 56 are mechanicallyinduced in the additional layer 54 after the additional layer is joinedto the outer layer 52. The perforations 56 are formed in a regularpattern of 9 mm holes spaced at 20-25 mm centres. The perforations maybe substituted with discrete discontinuous portions. Furthermore, anirregular pattern can be used with either perforations or discontinuousportions. For example, discrete discontinuous portions can be formed inan irregular pattern to the same effect as a regular pattern ofperforations, so long as the discontinuous portions are in at least 30%of the outer surface.

[0053] Generally, as the additional layer 54 increases in thickness, thesound barrier has more improved sound absorbing characteristics. Theminimum thickness of the additional layer 54 in this embodiment is 10mm. The maximum thickness of the additional layer 54 will depend on thesound frequencies being targeted by the sound barrier, although it isexpected that a maximum thickness is at about 25 mm.

[0054] The sound barrier of this embodiment has a density of about1650-1865 kg/m³. The outer layer 52 of the sound barrier has a densityof 1200-1700 kg/m³ whereas the additional layer 54 has a density ofabout 840-1200 kg/m³.

[0055] Reverberation chamber tests were conducted using three examplesof embodiments of the invention to compare their sound absorptioncharacteristics. Two of the examples (labelled S1 and S2) werestructurally the same as the sound barrier of FIG. 1; ie. having aninner and outer layer. The third example (labelled S3) was a soundbarrier made according to FIG. 5, having an inner layer, an outer layerand an additional layer, the additional layer being made from perforatedplasterboard material.

[0056]FIG. 6 is a chart showing the results of the tests for each of theabove sound barriers. As can be seen in FIG. 6, example S3 generally hasa higher sound absorption coefficient than examples S1 and S2 over thefrequency range, especially between about 400 Hz to 1400 Hz, due to thepresence of the perforations in the additional layer.

[0057] A method of making the sound barrier 10 in accordance to anotheraspect of the invention is also provided. A concrete mix is formedprimarily of RCA, the proportion of RCA varying between 45 and 60% ofthe concrete mix. Other materials can be included, such as generalpurpose (GP) cement, fly ash, concrete sand or water. The GP cement andfly ash is generally used as a binder for the section or panel. It hasbeen determined that the best results use a combination of the abovematerials. Various proportions of RCA and other materials in differentconcrete mixes are shown in Table 2 below for a panel with a compressivestrength of 25 MPa.

[0058] The concrete mix design is classified as a combination ofno-fines and gap-graded concrete and is described as “less-finesconcrete”. TABLE 2 Concret mix design Coarse RC Aggregate Fine RC WaterBinder Bulk Aggregate Concrete Vibration V Mass V Mass V Mass Air Mass VDensity time Panel Binder [m³] [kg] [m³] [kg] [m³] [kg] V [m³] [kg] [m³][kg/m³] [min] Alpha- GP+ 0.150 150 0.08 240.0 0.452 950 0.22 250 0.101,650 5.5 400 Fly 0.02 60.0 Ash Alpha- GP+ 0.150 150 0.08 240.0 0.474995 0.18 300 0.11 1,745 5.5 600 Fly 0.02 60.0 Ash Alpha- GP+ 0.150 1500.08 240.0 0.507 1065 0.13 350 0.13 1,865 5.5 800 Fly 0.02 60.0 Ash

[0059] The concrete mix designs in Table 2 are based on the followingcharacteristics:

[0060] GP cement having a specific gravity of 3.15;

[0061] fly ash having a specific gravity of 2.4;

[0062] coarse aggregate concrete with a specific gravity of 2.1 and abulk density of 1600 kg/m³;

[0063] fine aggregate concrete with a specific gravity of 2.58;

[0064] a slump of concrete of 50 mm (ie. the workability of freshconcrete into moulds to create the final shape of the set concrete);

[0065] a water/cement ratio of 0.55 (used in the mix and relates to thefinal compressive strength of the mix), and

[0066] fineness modulus (FM) of fine aggregate of 3.

[0067] FM is an aggregate property, which is the sum of cumulativeratios retained on 4.75 to 0.075 mm sieves. FM does not describe aparticle size distribution of the aggregate.

[0068] One manner in which the method according to the invention may beperformed is described below.

[0069] Once the appropriate concrete mix is selected, the mix is placedon a horizontal casting bed. The cast concrete mix is then compacted,preferably by using a vibrating table to promote the creation of voidsin the outer layer. The vibrating table is usually run at 50 Hz for 3.3to 5.5 minutes with amplitude of approximately 2 mm. An immersedvibrator may also be used to promote the creation of voids in the outerlayer.

[0070] After compaction, the concrete mix is then cured to set theconcrete mix as the sound barrier section. Curing may be performed usingeither steam curing or introducing a curing compound to the concretemix. Typical curing compounds include Duro-Seel, made by AbilityBuilding Chemicals.

[0071] Once the desired number of sections or panels have been produced,the sections are then joined or connected to each other to form soundbarrier 10. This can be done by sliding the sections into H-shape postsor attached to posts with bolts. Other methods can be used to connectthe sound barrier sections using standard structural engineering designand construction methods.

[0072] Table 3 shows several sound barriers made using according to theabove method using different concrete mixes, the manner of performingthe method only varying in the use of steam curing or a curing compound.TABLE 3 Concrete mix design and manufacturing method Concrete mix designRecycled Concrete Aggregate General 14/10 or Manufacturing method PanelPurpose 10 mm Curing/ type (GP) Pulverised Concrete Class 2 Placing/Compaction/ (SC) steam frequency [Hz]/ Cement Fuel Ash Sand IndustryHorizontal Vibrating curing/ compressive AS 1315 AS3582.1 AS 2758Standard Water casting table - 50 Hz (CC) curing strength [MPa] [kg][kg] [kg] [kg] [l] bed (3.3-5.5 min) compound α -400/25 MPa 240 60 250950 150 Yes Yes SC α -600/25 MPa 240 60 300 995 150 Yes Yes SC α -800/25MPa 240 60 350 1065 150 Yes Yes SC α -400/32 MPa 330 80 250 950 200 YesYes CC α -600/32 MPa 330 80 300 995 200 Yes Yes CC α -800/32 MPa 330 80350 1065 200 Yes Yes CC

[0073] In another example, about 57% RCA was used in the concrete mix tocreate a sound barrier having a compressive strength of 15 MPa, whichcan absorb frequencies of 400 Hz. Other sound barriers of compressivestrengths in the range of 25 MPa to 40 MPa were also be made.

[0074] For the embodiment described at FIG. 5, after the concrete mix isset to form the inner and outer layers, the additional layer is joinedto the outer layer. While the perforations or discontinuous portions canbe mechanically induced into the additional layer prior to joining, itis preferred that the perforations or discontinuous portions aremechanically induced in the additional layer after joining theadditional layer to the outer layer.

[0075] While the above example describes one particular manner ofperforming the method according to the invention, it is readily apparentto the person skilled in the art that other means may be employed toperform the casting, compacting and curing steps of the method accordingto the invention.

[0076] The above preferred embodiments and examples relate to a concretesound barrier for absorbing traffic noise generated along road surfaces.However, the sound barrier of the invention may also be applied to othersound reducing applications, such as for internal or external claddingin buildings.

[0077] It is understood that various modifications, alterations,variations and additions to the construction and arrangement of theembodiment described herein are considered as falling within the ambitand scope of the present invention.

The claims defining the invention are as follows:
 1. A sound barriermade from recycled concrete aggregate, the sound barrier being dividedinto at least two or more sections, each said section including: aninner layer for supporting the barrier, and an outer layer, said outerlayer having a predetermined number of voids for absorbing sound energyat a particular frequency, wherein said respective outer layers of saidat least two or more sections have different predetermined numbers ofvoids so as to absorb sound energy at different frequencies.
 2. A soundbarrier according to claim 1, wherein the outer layers have differentthicknesses to determine the number of voids in each section.
 3. A soundbarrier according to claim 2, wherein the thickness of said outer layersis up to 125 mm.
 4. A sound barrier according to claim 3, wherein saidthickness is no more than 85 mm.
 5. A sound barrier according to claim4, wherein said thickness is between 15 mm and 60 mm.
 6. A sound barrieraccording to claim 1, wherein said outer layers absorb frequencies inthe range of between 100 Hz and 2000 Hz.
 7. A sound barrier according toclaim 6, wherein said outer layers absorb frequencies between 400 Hz and800 Hz.
 8. A sound barrier according to claim 1, wherein there is atransition zone between the inner layer and the outer layer to reflectresidual sound energy that travels through the outer layer.
 9. A soundbarrier according to claim 8, wherein said inner layer is denser thansaid outer layer.
 10. A sound barrier according to claim 1, wherein thesound barrier has three sections arranged sequentially from top to baserelative to the ground in ascending order of each frequency absorbed.11. A sound barrier according to claim 1, wherein the voids include oneor more of the following types or a combination of the following types:gel-like pores; capillary-like pores; interconnected permeable voids;isolated air pockets or channels, and interconnected air pockets orchannels.
 12. A sound barrier according to claim 11, wherein the voidsrange in size between 100 Angstroms and 5 mm in diameter.
 13. A soundbarrier according to claim 1, wherein the outer layer is covered with anadditional layer, said additional layer including perforations ordiscontinuous portions to facilitate the absorption of sound energy. 14.A sound barrier according to claim 13, wherein the perforations ordiscontinuous portions extend into the additional layer to apredetermined depth.
 15. A sound barrier according to claim 14, whereinthe perforations or discontinuous portions extend into the additionallayer to a depth such that portions of the outer layer are exposed. 16.A sound barrier according to claim 13, wherein the perforations ordiscontinuous portions are formed over at least 30% of the outer surfaceof the additional layer.
 17. A sound barrier according to claim 13,wherein the additional layer is composed of a perforated material.
 18. Asound barrier according to claim 17, wherein the perforated materialincludes a mortar including RCA fines, cement, water and an airentraining agent.
 19. A sound barrier according to claim 17, wherein theperforated material includes plasterboard or render.
 20. A sound barrieraccording to claim 13, wherein the thickness of the additional layer isat least about 10 mm.
 21. A sound barrier according to claim 13, whereinthe thickness of the additional layer is at most about 25 mm.
 22. Asound barrier according to claim 13, wherein the perforations ordiscontinuous portions are formed in a regular pattern.
 23. A soundbarrier according to claim 13, wherein the perforations or discontinuousportions are mechanically induced in the additional layer.
 24. A methodof making a sound barrier divided into at least two or more sectionsfrom a concrete mix comprising recycled concrete aggregate, the methodcomprising the steps of: (a) casting the concrete mix; (b) compactingthe concrete mix to form a plurality of voids in the concrete mix; (c)curing the concrete mix to form one said section such that an outerlayer of said section has a predetermined number of said voids forabsorbing sound energy at a predetermined frequency; (d) repeating saidsteps (a)-(c) for one or more further sections wherein the number ofvoids in each respective outer layer differs so as to vary the frequencyof sound energy absorbed by each respective outer layer, and (e)connecting said section and said one or more further sections to formsaid sound barrier to absorb sound energy at different frequencies. 25.A method according to claim 24, wherein said number of voids in eachrespective outer layer is determined by varying the thickness of eachsaid respective outer layer.
 26. A method according to claim 24, whereinthe inner layer is formed with the outer layer of each section in asingle pour of concrete mix.
 27. A method according to claim 24, whereinthe concrete mix includes 45 to 60% of recycled concrete aggregate. 28.A method according to claim 24, wherein the recycled concrete aggregatehas a lower density than normal concrete.
 29. A method according toclaim 24, wherein the concrete mix includes one or more of thefollowing: general purpose cement; fly ash; concrete sand and water. 30.A method according to claim 24, wherein said casting step is performedon a horizontal casting bed.
 31. A method according to claim 24, whereinthe compacting step includes vibrating the concrete mix at apredetermined frequency and time.
 32. A method according to claim 31,wherein said predetermined frequency is about 50 Hz and saidpredetermined time is between 3 to 6 minutes.
 33. A method according toclaim 24, wherein said curing step is performed by steam curing or byintroducing a curing compound to the concrete mix.
 34. A methodaccording to claim 24, wherein each respective outer layer is covered byan additional layer including perforations or discontinuous portions tofacilitate the absorption of sound energy.
 35. A method according toclaim 34, wherein the perforations or discontinuous portions aremechanically induced in the additional layer.